1. Field of the Invention
This invention relates to an automatic controller for a film processor having a motor driven film transport roller arrangement and, in particular, to an automatic controller which utilizes both a position error signal and a velocity error signal to control the motor drive for the film transport rollers.
2. Description of the Prior Art
A film processor which includes coupled developing, fixing, washing and drying sections is well known. In such an apparatus, a film to be processed is introduced into the processor and is conveyed along a predetermined path through the processor by an arrangement of film transport rollers. The rollers are driven through a geared interconnection by a drive motor. Thus, the film is advanced on the rollers through the processor at a velocity that is functionally related to the velocity of rotation of the roller drive motor.
It is in the first two sections of the processor (developing and fixing) that the various chemical reactions occur which develop and fix the image of the exposed film. Due to the nature of the chemical reactions within the developing section of the processor, it is important to closely control the time interval (called "development time") during which the exposed film remains within the developing section of the processor. Much less criticality attaches to time that the film remains within the fixing, washing and drying sections. If the film should remain within the developing section for a period in excess of the development time, overdevelopment may occur. Conversely, the film may be underdeveloped if it remains in these sections for less than the desired development time. Both situations are not advantageous (if it is assumed that the development bath temperature and development chemical activity are within operative limits).
Since the portion of the predetermined path of the film that lies within the developing section of the processor is a fixed distance, and since the optimum development time for each film is known, it has been the practice to attempt to maintain the duration of film residency within the developing section to within predetermined close ranges of the development time by controlling the velocity at which the film is conveyed through the developing section. This velocity control for the film is usually accomplished by controlling the energy flow to the drive motor for the film transport rollers. The circuitry to effect this motor control function usually utilizes a signal, derived from a sensor disposed in proximity to a toothed wheel rotating in a functional relationship with the motor rotation, to generate a motor control feedback signal. The information derived from this sensor (which is representative of the film's position within the processor) is converted to a signal representative of measured film velocity. When the motor speed causes the film to deviate from the predetermined velocity, the motor control network operates to restore the film velocity to the predetermined velocity.
The rationality underlying the fixed velocity approach may be understood by reference to FIGS. 1A and 1B, which depict the ideal velocity-time and the distance-time relationships of known film processors. The reasoning underlying this technique relies upon the facts that the optimum or ideal development time T.sub.i is a known quantity, and that the distance D through which the film must be transported through the developing section is also known. Thus, if the motor is driven so as to transport the film at a constant, ideal velocity V.sub.i, after the expiration of the ideal development time T.sub.i, the film will have been transported the distance D through the developing section.
As a corollary to this principle, if, for whatever reason, the velocity at which the film is moving through the processor should deviate from the ideal velocity V.sub.i, appropriate corrective action is taken by the present motor drive control to return the film velocity to the ideal velocity V.sub.i. This response of the present motor drive control is graphically depicted in FIGS. 2A and 2B and FIGS. 3A and 3B, all of which depict approximations of the measured actual velocity-time and of the distance-time relationships for known film processors.
In the instance illustrated by the dotted points in FIG. 2A, the occurrence of some defect may cause a perturbation in the film transport velocity which increases the velocity of the film above the reference level V.sub.i. This effect is shown in the region of FIG. 2A indicated by reference character F. The motor control circuit associated with the drive motor derives an indication of this velocity increase from the toothed gear transducer and responds to the deviation by controlling the motor to cause the film velocity to return to the predetermined velocity V.sub.i, the correction being depicted in the region of FIG. 2A indicated by the reference character G. In some instances, a slight opposite deviation may occur, illustrated by the reference character H, but this overcompensation is usually relatively quickly damped by the system.
Another possible instance is illustrated by the starred points in FIG. 3A. If another perturbation occurs to decrease the actual velocity below the reference velocity V.sub.i (as in the region indicated at K in FIG. 3A), the velocity control arrangement derives an indication of this velocity decrease from the toothed gear transducer and acts so as to return the actual velocity toward the reference V.sub.i (as indicated in FIG. 3A at reference character L). Some overcompensation may occur, as at M, but this overcompensation is incidental to the response of the system and is relatively quickly damped. (Of course, it is understood that either or both types of perturbations may occur many times during the passage of any given film through the processor, and that the effect of the perturbation and the response of the prior motor control system are separately shown in FIGS. 2 and 3 for clarity of analysis.)
The effects of the perturbations in film velocity and of the actions of the motor control in response to these perturbations (regions F, G and H in FIG. 2A and regions K, L, and M in FIG. 3A) in terms of film residency in the development section are shown in FIGS. 2B and 3B, respectively.
In FIG. 2B, in the case of a velocity increasing perturbation (region F) the response of the motor control (regions G and, perhaps, H) is only to return the film velocity to the predetermined ideal velocity V.sub.i. As a result, however, the film reaches the distance D (i.e., it is traversed through the developing section) at the time T.sub.i -t.sub.1. And, at the time T.sub.i, the film has traversed a distance D+d.sub.1, where the distance D+d.sub.1 is beyond the developing section. Put alternately, since the film traversed the development distance D in a time less than the optimum development time T.sub.i, the film is likely to be underdeveloped.
Conversely, as is shown in FIG. 3B, in the case of an actual velocity decrease (region K) the response of the motor control (regions L and, perhaps, M) is again only to return the actual film velocity to the predetermined ideal velocity V.sub.i. As a consequence, at the time T.sub.i, the film has not yet traversed the full distance D but has been moved only through the distance D-d.sub.2. Stated alternately, the film will not traverse the full development distance D until a time T.sub.i +t.sub.2, which time is after the expiration of the optimum development time T.sub.i. Since the film remains within the developing section for a time longer than the optimum development time T.sub.i, the film is likely to be overdeveloped.
The disadvantages of overdevelopment and underdevelopment are believed to be caused by the response of the prior motor control systems in correcting only for velocity errors (deviations of measured actual velocity from the ideal velocity V.sub.i). Since it is critical to insure that the film occupy a position precisely at the exit of the developing section at precisely the idea time T.sub.i, and since with the prior (fixed velocity) motor control arrangements the deviation between the actual position of the film with respect to an ideal reference position (at any instant) goes uncorrected, it is believed that a fixed velocity motor control system as is used in the art is not totally desirable in the context of film processors. With such control systems, the increase or decrease in the actual position of the film within the developing section of the processor with respect to the ideal film position that the film should occupy were it not deviated from the ideal velocity goes uncompensated. Thus, although position information is available to the known film processors, this position information is not used when compensating for velocity perturbations.
It is believed to be advantageous to provide an automatic controller for a film processor which corrects for a velocity perturbation by not only returning the film velocity to the reference ideal velocity but also by returning the film to the ideal position it would have occupied but for the velocity perturbation so that optimum development time can be achieved. To accomplish this purpose, it is believed advantageous to generate a total motor error signal which is functionally related to both an error signal representative of the position error (representative of the difference in position between measured actual film position and an ideal reference position) and a velocity error signal (representative of the difference in actual film velocity and ideal film velocity). Further, it is believed to be advantageous to utilize the total motor error signal to generate a motor energy signal which may be applied to the motor to modify the amount of energy that is applied to the motor. Moreover, it is believed advantageous to periodically apply the motor energy signal in a manner that distributes the corrective action over a longer time period, rather than compensating for the effects of the velocity perturbation in the same time period as occupied by the perturbation. Although the invention may be implemented in both a hardwire analog or a hardwire digital mode, it is believed advantageous to practice the invention with a programmed digital computer, preferably a firmware-based, microcomputer arrangement.